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Friction and wear properties of hard coating materials on textured hard disk sliders
Wear 243 (2000) 133–139
Friction and wear properties of hard coating materials on textured hard disk sliders Lin Zhou a,∗ , Koji Kato a , Noritsugu Umehara b , Yoshihiko Miyake c b
a Laboratory of Tribology, School of Mechanical Engineering, Tohoku University, Sendai 980-8579, Japan Department of Mechanical Engineering, Tokyo Metropolitan Institute of Technology, 6-6, Hino, Asahigaoka 191-0065, Japan c Data Storage and Retrieval System Division, Odawara Works, Hitachi Ltd., Kozu, Odawara 256, Japan
Received 4 June 1999; received in revised form 3 May 2000; accepted 23 May 2000
Abstract It has been a common practice to apply a surface texture to disk surface in hard disk drive in order to minimize the stiction force caused by the contact of extremely smooth surfaces of head sliders and disks. The slider surface texture was also suggested recently with the advantage of using smoother disks. The slider surface texture reduced the stiction effect, meanwhile it increased the contact pressure between sliders and disks. The protective coating is required on the textured slider surface to reduce the wear of the texture. In the present research, four kinds of hard coating materials were synthesized by IBAD and evaluated on textured slider surfaces by drag test. The results showed that the a-CNx coating had higher wear resistance than sputtered carbon, TiNx and BNx coatings on textured slider surfaces. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Wear; Coating; Texture; Hard disk slider
1. Introduction The high storage density required in modern hard disk drives can be achieved by reducing the flying height of read/write head sliders over surfaces of thin-film disk [1]. Although disks with extremely flat surface are desirable to ensure a stable, low flying height, such disks usually tend to show high friction caused by stiction during start/stop operations, which is undesirable when the durability of magnetic storage system is concerned. For this reason, it has been a common practice to apply a surface texture to disks in order to minimize the stiction force. A different way of implementing a textured head/disk interface is by texturing the slider rather than the disk. The advantage lies in the possibility of using smoother disks. In 1997, Zhou et al. proposed a new texturing method for head slider surface by ion beam etching [2,3]. The results showed that the texture on head slider surface could significantly reduce the friction between head slider and disk under contact condition. The problem lies there is the insufficient wear resistance of the texture. It is necessary to deposit a wear resistant coating on textured slider surface as protective overcoat. ∗ Corresponding author. Tel.: +81-22-217-6957; fax: +81-22-217-6955. E-mail address: [email protected] (L. Zhou).
The common coating material for hard disk surface is diamond-like-carbon (DLC). According to theoretical prediction [4,5], crystalline carbon nitride (-C3 N4 ) can even be harder than diamond. Based on this prediction, a newly developed coating material, amorphous carbon nitride (hereafter referred to as a-CNx ) attracted much research attention. The a-CNx has become a strong competitor for DLC as wear resistant coating material [6]. In the present paper, ion beam assisted deposition (IBAD) was used to synthesize a-CNx thin films as well as sputtered carbon, TiNx , BNx on textured sliders as protective coatings. The friction and wear properties of these head sliders against DLC-coated hard disks were evaluated by a constant speed drag test. 2. Experimental procedure 2.1. Coating deposition An IBAD system was used for texturing surfaces of head sliders and depositing coatings on the textured slider surfaces. The IBAD system (Fig. 1) was developed by Hitachi Ltd., Japan. It consists of a cryogenically pumped chamber, a sputter deposition source, a bucket-type ion source and a substrate holder. The diameter of the ion beam irradiation
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2.2. Topography, hardness and chemical composition
Fig. 1. Ion beam assisted deposition system.
area is about 80 mm. The substrate holder consists of a water-cooled copper plate and rotates at 4 rpm during the deposition. The sputtered carbon (hereafter referred to as a-C) coatings were deposited using an argon ion beam to sputtering utilizing a 99.999% pure carbon target. The energy of the argon ions for sputtering carbon was 1.0 keV and the ion current was 100 mA. The deposition rate was 1.4 nm min−1 . The a-CNx coatings, BNx coatings and TiNx coatings were deposited by using the argon ion beam to sputter carbon, boron or titanium and a simultaneous bombardment of nitrogen ion beam. The energy of the assisted nitrogen ion beam was fixed at 1.0 keV for the depositions of the a-CNx coatings, BNx coatings and TiNx coatings. The ion current density varied for finding the optimum deposition conditions of different kinds of coatings. The deposition parameters are listed in Table 1. The four coating materials were then deposited on the textured sliders with a thickness of 50 nm for the evaluation of coating wear resistance. The yttria stabilized zirconia (Al2 O3 5 wt.%, TiC 5 wt.%) hard disk sliders were textured by ion beam etching [7]. The height of island-typed texture was 13.3 nm.
All coatings deposited on silicon wafers were analyzed by a nano-indenter (NEC MHA-400). The thickness of all tested coatings was about 100 nm. The thickness of coatings was determined by measuring the step height of a masked specimen with AFM. The hardness values of the coatings were obtained from the slopes. The load was adjusted within 0–0.5 mN to get a penetration depth of 30 nm. The indentation speed was 2.7 nm s−1 . All hardness tests were done with the same diamond tip under the same test conditions. The depth distribution of chemical composition of the a-C coating and the three hardest coatings of the a-CNx coatings, the BNx coatings and the TiNx coatings were selected to be analyzed by XPS(ULVAC PHI ESCA-5500). The analysis was done with an Al K␣ (1486.5 eV) X-ray source. The Raman spectrum was applied to analyze the chemical structure of the a-CNx and a-C coatings on silicon wafers. 2.3. Friction and wear test The textured and coated sliders were tested in contact sliding against bump-type-textured hard disks to evaluate the wear resistance of the protective coatings. The disks were unlubricated and DLC coated. The schematic of the slider-on-disk tribo-tester is shown in Fig. 2. The load between slider and disk was applied by the elastic deformation of the leaf spring. The used load was 80 mN. This is the same as the load used in hard disk drives of this kind of sliders. Friction force and load were continuously recorded during the test by the strain gages on the leaf springs. All tests were done with new disks at the same track radius to keep the same sliding distance of the same sliding cycles. In
Table 1 Deposition parameters of coatings
Current density of N ion beam for mixing (A cm−2 )
Current of Ar ion beam for sputtering Energy of N ion beam for mixing Energy of Ar ion beam for sputtering
a-CNx
DLC
BNx
TiNx
10
–
17
16
20 30 40
– – –
35 69 85
31 63 83 100
100 mA 1 keV 1 keV Fig. 2. Schematic of drag test equipment.
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135
order to accelerate the tests, the sliding speed was controlled at 0.5 m s−1 . It kept sliders and disks in contact during the sliding test. Tests were stopped at 20,000 cycles or when the disk failed (when a faint wear track was observed by naked eye on the surface of the disk). The drag tests were carried out in air with a humidity of RH45–50%. The tested sliders and disks were observed with an AFM (SPA300 series, Seiko Instruments Inc., Japan). The wear of slider surfaces could be found out by measuring the remained height of the islands on the head slider air-bearing surface.
3. Experimental results
Fig. 4. XPS depth analysis of a-CNx coating on Si wafer.
3.1. Coating hardness The hardness results of coatings deposited at different current densities of the nitrogen ion beam are shown in Fig. 3. The a-CNx coating had a peak hardness value of 21.1 GPa at 0.2 mA cm−2 , the BNx coating had a peak hardness value of 10.4 GPa at 0.7 mA cm−2 and the TiNx coating had a peak hardness value of 11.2 GPa at 0.8 mA cm−2 . The a-CNx coating was about two times harder than the BNx coating and the TiNx coating. 3.2. Chemical compositions 3.2.1. XPS Figs. 4–7 shows the XPS results of atomic concentrations of elements along the depth direction of the a-CNx , the a-C, the BNx and the TiNx coating on Si wafers. The concentration of nitrogen is 11% for the a-CNx coating and 48% for the BNx coating and the TiNx costing. The XPS result also shows that there are mixed interlayers between coatings and substrates that give enough adhesive strength between the coatings and the substrates.
Fig. 3. The effect of nitrogen ion beam current density on the hardness of coatings.
Fig. 5. XPS depth analysis of a-C coating on Si wafer.
3.2.2. Raman spectroscopy The Raman spectrums of the a-CNx and a-C coatings on silicon wafers are shown in Fig. 8(a) and (b). The Gaussian lineshape fit is also shown in the figure. Raman spectra are sensitive to changes in translational symmetry and are, thus, useful for the study of disorder or crystallite formation and of structural modifications in carbon-based films [8]. The first order Raman spectrum for
Fig. 6. XPS depth analysis of BNx coating on Si wafer.
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Fig. 7. XPS depth analysis of TiNx coating on Si wafer.
diamond and graphite consists of a single line at 1332 cm−1 and at about 1580 cm−1 (G), respectively. In disordered graphite, where the long-range-translational symmetry is lost, crystal momentum need no longer be conserved and the spectrum changes drastically. The most striking effect is that in addition to the G band, another disorder or ‘D’ broad line appears at about 1355 cm−1 , which is also associated with in-plane vibrations. Raman spectra of the a-C coating and the a-CNx coating are characterized by two Guassian broad shoulder lines at about 1555 and 1370 cm−1 . The one
Fig. 9. Friction coefficient between coated slider and bump-type textured disk in relation to sliding cycles.
at 1370 cm−1 is called D band and the other at 1555 cm−1 is called G band. 3.3. Friction and wear The drag test results are shown in Fig. 9. All tests are stopped after 20,000 cycles (about 2800 m) of sliding, except the test of the TiNx coated slider. After 8000 cycles, the coefficient of friction of the TiNx coated slider against the disk reached a high value (>1.0), and a clear wear track could be observed by naked eye on the disk surface. Therefore, the test was stopped at 10,000 cycles of sliding for the TiNx coated slider. From Fig. 9, the a-CNx and the a-C coated sliders had stable coefficient of friction of 0.3. The coefficient of friction of the BNx coated slider increased slightly during the test, and had a final value of 0.41. The wear of the disks for the a-CNx , the a-C and the BNx coated sliders was so small that it is impossible to be measured by the profilometer. These disks were also observed under the AFM. The wear amount of the disk surface was difficult to be measured, because the amount of wear was within the scatter range of the bump height on the disk surface. The surfaces of tested hard sliders were observed with the AFM. The AFM images are shown in Fig. 10(a)–(d). The wear amounts of the head slider surfaces were evaluated by measuring the remained height of the islands on the slider surfaces. The wear volumes can be then calculated with the slider surface bearing curves. The wear rates of the sliders with four kinds of coatings are shown in Fig. 11. 4. Discussions 4.1. The effect of coating hardness on the slider wear and the friction
Fig. 8. Raman results of coatings on Si wafer (a) a-CNx coating (b) a-C coating.
The initial coefficient of friction of sliders with different coating materials are almost the same (about 0.2) (Fig. 9). It
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Fig. 10. AFM images of tested slider surfaces of different coatings (a) a-CNx coating; (b) a-C coating; (c) BNx coating; (d) TiNx coating.
suggests that there is no material effect on friction between sliders and disks for the four kinds of selected coating materials. The later change of the coefficient of friction is considered to be due to the change of slider surface topography. The wear of slider surface texture results in the increasing contact area between sliders and disks, and as a result, the coefficient of friction increased. The texture (islands) on the TiNx and the BNx coated head sliders was worn out after the drag tests. The surface of the TiNx coated slider became flat after the test (Fig. 10(d)) and the texture on the disk surface was also worn (Fig. 10(b)). Thus, the friction increased to a high value of over 1.0. In the case of the BNx coated head slider, wear particles were trapped on the head slider surface (Fig. 10(c)). Many bumps were formed by these wear particles. These bumps reduced the contact area between the head slider and disk in the test. The friction between the BNx coated head slider and disk did not increase as much as the TiNx coated slider. The remained height of the islands on the a-CNx coated slider was larger than that of the a-C coated slider (Fig. 10(a) and (b)). In both cases, the remained islands on slider
surfaces were enough to separate the surfaces of the head slider and disk. The coefficient of friction of the a-C coated slider and the a-CNx coated slider was almost the same. The most important property of protective coating on textured slider is considered to be hardness. Harder coating results in a lower wear rate (Fig. 12). As a wear resistant coating material for textured head sliders, the a-CNx is better than the a-C, the BNx and the TiNx . In summary, the texture on slider surface is essential to lower and stablilise friction between slider and disk. In order to protect the texture from wear, high wear resistant overcoat is necessary on the textured slider surface.
4.2. The effect of chemical properties on the hardness of the a-CNx coating The atomic concentration of nitrogen in the a-CNx coating was 10%. It is lower than the theoretically predicted concentration of -C3 N4 . The structure of the coating was amorphous. Not every carbon atoms compounded with four
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Fig. 10. (Continued).
nitrogen atoms. In the IBAD deposition process, there is an optimum ion density value for the best wear resistance at a certain ion energy level. Since, there are too many bombardment damages inside the coating above the value, these damages reduced the wear resistance. However, on the other hand, there is not enough chemical compound formed to
improve the wear resistance of the coating below this optimum ion density value. In the Raman analysis of carbon-based coatings, the intensity of the D band (ID ) has been found to be related to the crystallite size or the short-range cluster size in the coating [9]. The presence of nitrogen in the a-CNx coat-
Fig. 11. Wear of slider surfaces with different coatings in sliding against bump-type textured disk.
Fig. 12. The relationship between slider surface wear and coating hardness.
L. Zhou et al. / Wear 243 (2000) 133–139
ing somehow increases the crystallization in the local area. Resonant Raman experiments [10] and IR spectroscopy result [11] from the DLC films have shown that Raman spectra can be described by a heterogeneous structural model involving two phases (differing in their electronic and vibronic properties). According to this model [10], the high-frequency band in DLC Raman spectra is assigned to a graphite-like sp2 -bonded phase and the low-frequency band to the sp3 -bonded phase [8]. According to this model, the a-CNx coating contains more sp3 -bonded carbon than the a-C coating. The values of ID /IG are 1.21 and 4.20 for the a-C and the a-CNx coatings, respectively. This result suggests that the a-CNx coating contains more sp3 -bonding than the a-C coating. Thus, the a-CNx coating is harder than the a-C coating. It is considered to be the reason that the a-CNx coating has higher wear resistance than the a-C coating as protective coating on textured head slider surfaces.
5. Conclusions In the present research, four kinds of hard coating materials were synthesized by IBAD. Their friction and wear properties on textured hard disk sliders were evaluated by constant speed drag test. The following conclusions are obtained: 1. a-CNx is better than the a-C, BNx and TiNx for its higher wear resistance and lower coefficient of friction. 2. The coefficient of friction between the a-CNx or the a-C coated slider against the DLC coated unlubricated disk (µ=0.3) was lower than that of BNx (µ=0.4) or TiNx (µ=1.0) coated slider.
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3. The wear rate of the a-CNx coated slider is half of that of the a-C coated slider, one-fifth of that of the BNx coated slider and one-tenth of that of the TiNx coated slider. References [1] B. Bhushan, Tribology and Mechanics of Magnetic Storage Device, 2nd Edition, Springer, New York, 1996, pp. 14–59. [2] L. Zhou, K. Kato, N. Umehara, Y. Miyake, Tribological properties of hard disk slider overcoated after ion beam texturing, in: Proceedings of the International Conference on Micro-Mechatronics for Information and Precision Equipment, Tokyo, 1997, pp. 377–382. [3] L. Zhou, K. Kato, N. Umehara, Y. Miyake, Tribological properties of hard disk slider overcoated after ion beam texturing, Adv. Info. Storage Syst. 9 (1998) 263–275. [4] A.Y. Liu, M.L. Cohen, Prediction of new low compressibility solids, Science 37 (1989) 479–482. [5] A.Y. Liu, M.L. Cohen, Structural properties and electronic structure of low-compressibility materials: -Si3 N4 and hypothetical -C3 N4 , Phys. Rev. B41 (1990) 10727–10734. [6] A. Khurshudov, K. Kato, D. Sawada, Microtribological characterization of carbon nitride coatings, in: Proceedings of the International Tribology Conference, Yokohama, Japan, 1995, pp. 1931–1936. [7] L. Zhou, K. Kato, N. Umehara, Y. Miyake, Nanometer scale islandtype texture with controllable height and area ratio formed by ionbeam etching on hard-disk head sliders, Nanotechnology 10 (1999) 363–372. [8] V. Palshin, E.I. Meletis, S. Ves, S. Logothetidis, Characterization of ion-beam-deposited diamond-like carbon films, Thin Solid Films 270 (1995) 165–172. [9] F. Tuinstra, J.L. Koening, Characterization of graphite fiber surface with Raman spectroscopy, J. Compos. Mater. 4 (1970) 492–499. [10] J. Wagner, M. Ramsteiner, C. Wild, P. Koidl, Resonant Raman scattering of amorphous carbon and polycrystalline diamond films, Phys. Rev. B 40 (3) (1989) 1817–1824. [11] B. Dischler, A. Bubenzer, P. Koidl, Bonding in hydrogenated hard carbon studied by optical spectroscopy, Solid State Commun. 48 (2) (1983) 105–108.
Friction and wear properties of hard coating materials on textured hard disk sliders Lin Zhou a,∗ , Koji Kato a , Noritsugu Umehara b , Yoshihiko Miyake c b
a Laboratory of Tribology, School of Mechanical Engineering, Tohoku University, Sendai 980-8579, Japan Department of Mechanical Engineering, Tokyo Metropolitan Institute of Technology, 6-6, Hino, Asahigaoka 191-0065, Japan c Data Storage and Retrieval System Division, Odawara Works, Hitachi Ltd., Kozu, Odawara 256, Japan
Received 4 June 1999; received in revised form 3 May 2000; accepted 23 May 2000
Abstract It has been a common practice to apply a surface texture to disk surface in hard disk drive in order to minimize the stiction force caused by the contact of extremely smooth surfaces of head sliders and disks. The slider surface texture was also suggested recently with the advantage of using smoother disks. The slider surface texture reduced the stiction effect, meanwhile it increased the contact pressure between sliders and disks. The protective coating is required on the textured slider surface to reduce the wear of the texture. In the present research, four kinds of hard coating materials were synthesized by IBAD and evaluated on textured slider surfaces by drag test. The results showed that the a-CNx coating had higher wear resistance than sputtered carbon, TiNx and BNx coatings on textured slider surfaces. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Wear; Coating; Texture; Hard disk slider
1. Introduction The high storage density required in modern hard disk drives can be achieved by reducing the flying height of read/write head sliders over surfaces of thin-film disk [1]. Although disks with extremely flat surface are desirable to ensure a stable, low flying height, such disks usually tend to show high friction caused by stiction during start/stop operations, which is undesirable when the durability of magnetic storage system is concerned. For this reason, it has been a common practice to apply a surface texture to disks in order to minimize the stiction force. A different way of implementing a textured head/disk interface is by texturing the slider rather than the disk. The advantage lies in the possibility of using smoother disks. In 1997, Zhou et al. proposed a new texturing method for head slider surface by ion beam etching [2,3]. The results showed that the texture on head slider surface could significantly reduce the friction between head slider and disk under contact condition. The problem lies there is the insufficient wear resistance of the texture. It is necessary to deposit a wear resistant coating on textured slider surface as protective overcoat. ∗ Corresponding author. Tel.: +81-22-217-6957; fax: +81-22-217-6955. E-mail address: [email protected] (L. Zhou).
The common coating material for hard disk surface is diamond-like-carbon (DLC). According to theoretical prediction [4,5], crystalline carbon nitride (-C3 N4 ) can even be harder than diamond. Based on this prediction, a newly developed coating material, amorphous carbon nitride (hereafter referred to as a-CNx ) attracted much research attention. The a-CNx has become a strong competitor for DLC as wear resistant coating material [6]. In the present paper, ion beam assisted deposition (IBAD) was used to synthesize a-CNx thin films as well as sputtered carbon, TiNx , BNx on textured sliders as protective coatings. The friction and wear properties of these head sliders against DLC-coated hard disks were evaluated by a constant speed drag test. 2. Experimental procedure 2.1. Coating deposition An IBAD system was used for texturing surfaces of head sliders and depositing coatings on the textured slider surfaces. The IBAD system (Fig. 1) was developed by Hitachi Ltd., Japan. It consists of a cryogenically pumped chamber, a sputter deposition source, a bucket-type ion source and a substrate holder. The diameter of the ion beam irradiation
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2.2. Topography, hardness and chemical composition
Fig. 1. Ion beam assisted deposition system.
area is about 80 mm. The substrate holder consists of a water-cooled copper plate and rotates at 4 rpm during the deposition. The sputtered carbon (hereafter referred to as a-C) coatings were deposited using an argon ion beam to sputtering utilizing a 99.999% pure carbon target. The energy of the argon ions for sputtering carbon was 1.0 keV and the ion current was 100 mA. The deposition rate was 1.4 nm min−1 . The a-CNx coatings, BNx coatings and TiNx coatings were deposited by using the argon ion beam to sputter carbon, boron or titanium and a simultaneous bombardment of nitrogen ion beam. The energy of the assisted nitrogen ion beam was fixed at 1.0 keV for the depositions of the a-CNx coatings, BNx coatings and TiNx coatings. The ion current density varied for finding the optimum deposition conditions of different kinds of coatings. The deposition parameters are listed in Table 1. The four coating materials were then deposited on the textured sliders with a thickness of 50 nm for the evaluation of coating wear resistance. The yttria stabilized zirconia (Al2 O3 5 wt.%, TiC 5 wt.%) hard disk sliders were textured by ion beam etching [7]. The height of island-typed texture was 13.3 nm.
All coatings deposited on silicon wafers were analyzed by a nano-indenter (NEC MHA-400). The thickness of all tested coatings was about 100 nm. The thickness of coatings was determined by measuring the step height of a masked specimen with AFM. The hardness values of the coatings were obtained from the slopes. The load was adjusted within 0–0.5 mN to get a penetration depth of 30 nm. The indentation speed was 2.7 nm s−1 . All hardness tests were done with the same diamond tip under the same test conditions. The depth distribution of chemical composition of the a-C coating and the three hardest coatings of the a-CNx coatings, the BNx coatings and the TiNx coatings were selected to be analyzed by XPS(ULVAC PHI ESCA-5500). The analysis was done with an Al K␣ (1486.5 eV) X-ray source. The Raman spectrum was applied to analyze the chemical structure of the a-CNx and a-C coatings on silicon wafers. 2.3. Friction and wear test The textured and coated sliders were tested in contact sliding against bump-type-textured hard disks to evaluate the wear resistance of the protective coatings. The disks were unlubricated and DLC coated. The schematic of the slider-on-disk tribo-tester is shown in Fig. 2. The load between slider and disk was applied by the elastic deformation of the leaf spring. The used load was 80 mN. This is the same as the load used in hard disk drives of this kind of sliders. Friction force and load were continuously recorded during the test by the strain gages on the leaf springs. All tests were done with new disks at the same track radius to keep the same sliding distance of the same sliding cycles. In
Table 1 Deposition parameters of coatings
Current density of N ion beam for mixing (A cm−2 )
Current of Ar ion beam for sputtering Energy of N ion beam for mixing Energy of Ar ion beam for sputtering
a-CNx
DLC
BNx
TiNx
10
–
17
16
20 30 40
– – –
35 69 85
31 63 83 100
100 mA 1 keV 1 keV Fig. 2. Schematic of drag test equipment.
L. Zhou et al. / Wear 243 (2000) 133–139
135
order to accelerate the tests, the sliding speed was controlled at 0.5 m s−1 . It kept sliders and disks in contact during the sliding test. Tests were stopped at 20,000 cycles or when the disk failed (when a faint wear track was observed by naked eye on the surface of the disk). The drag tests were carried out in air with a humidity of RH45–50%. The tested sliders and disks were observed with an AFM (SPA300 series, Seiko Instruments Inc., Japan). The wear of slider surfaces could be found out by measuring the remained height of the islands on the head slider air-bearing surface.
3. Experimental results
Fig. 4. XPS depth analysis of a-CNx coating on Si wafer.
3.1. Coating hardness The hardness results of coatings deposited at different current densities of the nitrogen ion beam are shown in Fig. 3. The a-CNx coating had a peak hardness value of 21.1 GPa at 0.2 mA cm−2 , the BNx coating had a peak hardness value of 10.4 GPa at 0.7 mA cm−2 and the TiNx coating had a peak hardness value of 11.2 GPa at 0.8 mA cm−2 . The a-CNx coating was about two times harder than the BNx coating and the TiNx coating. 3.2. Chemical compositions 3.2.1. XPS Figs. 4–7 shows the XPS results of atomic concentrations of elements along the depth direction of the a-CNx , the a-C, the BNx and the TiNx coating on Si wafers. The concentration of nitrogen is 11% for the a-CNx coating and 48% for the BNx coating and the TiNx costing. The XPS result also shows that there are mixed interlayers between coatings and substrates that give enough adhesive strength between the coatings and the substrates.
Fig. 3. The effect of nitrogen ion beam current density on the hardness of coatings.
Fig. 5. XPS depth analysis of a-C coating on Si wafer.
3.2.2. Raman spectroscopy The Raman spectrums of the a-CNx and a-C coatings on silicon wafers are shown in Fig. 8(a) and (b). The Gaussian lineshape fit is also shown in the figure. Raman spectra are sensitive to changes in translational symmetry and are, thus, useful for the study of disorder or crystallite formation and of structural modifications in carbon-based films [8]. The first order Raman spectrum for
Fig. 6. XPS depth analysis of BNx coating on Si wafer.
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Fig. 7. XPS depth analysis of TiNx coating on Si wafer.
diamond and graphite consists of a single line at 1332 cm−1 and at about 1580 cm−1 (G), respectively. In disordered graphite, where the long-range-translational symmetry is lost, crystal momentum need no longer be conserved and the spectrum changes drastically. The most striking effect is that in addition to the G band, another disorder or ‘D’ broad line appears at about 1355 cm−1 , which is also associated with in-plane vibrations. Raman spectra of the a-C coating and the a-CNx coating are characterized by two Guassian broad shoulder lines at about 1555 and 1370 cm−1 . The one
Fig. 9. Friction coefficient between coated slider and bump-type textured disk in relation to sliding cycles.
at 1370 cm−1 is called D band and the other at 1555 cm−1 is called G band. 3.3. Friction and wear The drag test results are shown in Fig. 9. All tests are stopped after 20,000 cycles (about 2800 m) of sliding, except the test of the TiNx coated slider. After 8000 cycles, the coefficient of friction of the TiNx coated slider against the disk reached a high value (>1.0), and a clear wear track could be observed by naked eye on the disk surface. Therefore, the test was stopped at 10,000 cycles of sliding for the TiNx coated slider. From Fig. 9, the a-CNx and the a-C coated sliders had stable coefficient of friction of 0.3. The coefficient of friction of the BNx coated slider increased slightly during the test, and had a final value of 0.41. The wear of the disks for the a-CNx , the a-C and the BNx coated sliders was so small that it is impossible to be measured by the profilometer. These disks were also observed under the AFM. The wear amount of the disk surface was difficult to be measured, because the amount of wear was within the scatter range of the bump height on the disk surface. The surfaces of tested hard sliders were observed with the AFM. The AFM images are shown in Fig. 10(a)–(d). The wear amounts of the head slider surfaces were evaluated by measuring the remained height of the islands on the slider surfaces. The wear volumes can be then calculated with the slider surface bearing curves. The wear rates of the sliders with four kinds of coatings are shown in Fig. 11. 4. Discussions 4.1. The effect of coating hardness on the slider wear and the friction
Fig. 8. Raman results of coatings on Si wafer (a) a-CNx coating (b) a-C coating.
The initial coefficient of friction of sliders with different coating materials are almost the same (about 0.2) (Fig. 9). It
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Fig. 10. AFM images of tested slider surfaces of different coatings (a) a-CNx coating; (b) a-C coating; (c) BNx coating; (d) TiNx coating.
suggests that there is no material effect on friction between sliders and disks for the four kinds of selected coating materials. The later change of the coefficient of friction is considered to be due to the change of slider surface topography. The wear of slider surface texture results in the increasing contact area between sliders and disks, and as a result, the coefficient of friction increased. The texture (islands) on the TiNx and the BNx coated head sliders was worn out after the drag tests. The surface of the TiNx coated slider became flat after the test (Fig. 10(d)) and the texture on the disk surface was also worn (Fig. 10(b)). Thus, the friction increased to a high value of over 1.0. In the case of the BNx coated head slider, wear particles were trapped on the head slider surface (Fig. 10(c)). Many bumps were formed by these wear particles. These bumps reduced the contact area between the head slider and disk in the test. The friction between the BNx coated head slider and disk did not increase as much as the TiNx coated slider. The remained height of the islands on the a-CNx coated slider was larger than that of the a-C coated slider (Fig. 10(a) and (b)). In both cases, the remained islands on slider
surfaces were enough to separate the surfaces of the head slider and disk. The coefficient of friction of the a-C coated slider and the a-CNx coated slider was almost the same. The most important property of protective coating on textured slider is considered to be hardness. Harder coating results in a lower wear rate (Fig. 12). As a wear resistant coating material for textured head sliders, the a-CNx is better than the a-C, the BNx and the TiNx . In summary, the texture on slider surface is essential to lower and stablilise friction between slider and disk. In order to protect the texture from wear, high wear resistant overcoat is necessary on the textured slider surface.
4.2. The effect of chemical properties on the hardness of the a-CNx coating The atomic concentration of nitrogen in the a-CNx coating was 10%. It is lower than the theoretically predicted concentration of -C3 N4 . The structure of the coating was amorphous. Not every carbon atoms compounded with four
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Fig. 10. (Continued).
nitrogen atoms. In the IBAD deposition process, there is an optimum ion density value for the best wear resistance at a certain ion energy level. Since, there are too many bombardment damages inside the coating above the value, these damages reduced the wear resistance. However, on the other hand, there is not enough chemical compound formed to
improve the wear resistance of the coating below this optimum ion density value. In the Raman analysis of carbon-based coatings, the intensity of the D band (ID ) has been found to be related to the crystallite size or the short-range cluster size in the coating [9]. The presence of nitrogen in the a-CNx coat-
Fig. 11. Wear of slider surfaces with different coatings in sliding against bump-type textured disk.
Fig. 12. The relationship between slider surface wear and coating hardness.
L. Zhou et al. / Wear 243 (2000) 133–139
ing somehow increases the crystallization in the local area. Resonant Raman experiments [10] and IR spectroscopy result [11] from the DLC films have shown that Raman spectra can be described by a heterogeneous structural model involving two phases (differing in their electronic and vibronic properties). According to this model [10], the high-frequency band in DLC Raman spectra is assigned to a graphite-like sp2 -bonded phase and the low-frequency band to the sp3 -bonded phase [8]. According to this model, the a-CNx coating contains more sp3 -bonded carbon than the a-C coating. The values of ID /IG are 1.21 and 4.20 for the a-C and the a-CNx coatings, respectively. This result suggests that the a-CNx coating contains more sp3 -bonding than the a-C coating. Thus, the a-CNx coating is harder than the a-C coating. It is considered to be the reason that the a-CNx coating has higher wear resistance than the a-C coating as protective coating on textured head slider surfaces.
5. Conclusions In the present research, four kinds of hard coating materials were synthesized by IBAD. Their friction and wear properties on textured hard disk sliders were evaluated by constant speed drag test. The following conclusions are obtained: 1. a-CNx is better than the a-C, BNx and TiNx for its higher wear resistance and lower coefficient of friction. 2. The coefficient of friction between the a-CNx or the a-C coated slider against the DLC coated unlubricated disk (µ=0.3) was lower than that of BNx (µ=0.4) or TiNx (µ=1.0) coated slider.
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3. The wear rate of the a-CNx coated slider is half of that of the a-C coated slider, one-fifth of that of the BNx coated slider and one-tenth of that of the TiNx coated slider. References [1] B. Bhushan, Tribology and Mechanics of Magnetic Storage Device, 2nd Edition, Springer, New York, 1996, pp. 14–59. [2] L. Zhou, K. Kato, N. Umehara, Y. Miyake, Tribological properties of hard disk slider overcoated after ion beam texturing, in: Proceedings of the International Conference on Micro-Mechatronics for Information and Precision Equipment, Tokyo, 1997, pp. 377–382. [3] L. Zhou, K. Kato, N. Umehara, Y. Miyake, Tribological properties of hard disk slider overcoated after ion beam texturing, Adv. Info. Storage Syst. 9 (1998) 263–275. [4] A.Y. Liu, M.L. Cohen, Prediction of new low compressibility solids, Science 37 (1989) 479–482. [5] A.Y. Liu, M.L. Cohen, Structural properties and electronic structure of low-compressibility materials: -Si3 N4 and hypothetical -C3 N4 , Phys. Rev. B41 (1990) 10727–10734. [6] A. Khurshudov, K. Kato, D. Sawada, Microtribological characterization of carbon nitride coatings, in: Proceedings of the International Tribology Conference, Yokohama, Japan, 1995, pp. 1931–1936. [7] L. Zhou, K. Kato, N. Umehara, Y. Miyake, Nanometer scale islandtype texture with controllable height and area ratio formed by ionbeam etching on hard-disk head sliders, Nanotechnology 10 (1999) 363–372. [8] V. Palshin, E.I. Meletis, S. Ves, S. Logothetidis, Characterization of ion-beam-deposited diamond-like carbon films, Thin Solid Films 270 (1995) 165–172. [9] F. Tuinstra, J.L. Koening, Characterization of graphite fiber surface with Raman spectroscopy, J. Compos. Mater. 4 (1970) 492–499. [10] J. Wagner, M. Ramsteiner, C. Wild, P. Koidl, Resonant Raman scattering of amorphous carbon and polycrystalline diamond films, Phys. Rev. B 40 (3) (1989) 1817–1824. [11] B. Dischler, A. Bubenzer, P. Koidl, Bonding in hydrogenated hard carbon studied by optical spectroscopy, Solid State Commun. 48 (2) (1983) 105–108.